1. Field of the Invention
This invention related to a picture signal encoding apparatus for compression-coding picture signals and a picture signal decoding apparatus for expansion-decoding compression-coded picture signals.
2. Description of the Related Art
A digital video tape recorder (VTR) is used as means for recording/reproducing picture signals of a television broadcasting program or the like. There are several formats of the digital VTR, such as, the d1 standard for program production and the d2 standard for broadcasting stations. With respect to such digital VTR for program production or for broadcasting stations, since extremely high quality is required of pictures to be recorded/reproduced, it is normal that picture signals to be recorded onto a magnetic tape are not compressed, or that a relatively low compression rate for compressing the data quantity to approximately 1/2 is used.
Meanwhile, the digital VTR for commercial use is becoming broadly used. The digital VTR for commercial use needs to enable long-time recording using a compact cassette tape, though the requirement of picture quality is lower than that of the digital VTR for business use. Therefore, a compression format for performing band compression at a relatively high compression rate for compressing the picture data quantity to approximately 1/5 is employed. The compression format of the digital VTR for commercial use provides high picture quality, and therefore used also for video equipments for broadcasting business. The high-efficiency compression method used here utilizes the fact that the human visual characteristic is less sensitive to color-difference information than to luminance information of a picture, and therefore, the information amount of color-difference signals is reduced. This picture data compression method will be described later.
In accordance with the above-described D1 standard as a digital VTR standard for program production or for broadcasting stations, digital picture data to be recorded onto a magnetic tape are not compressed, and a luminance signal Y and color-difference signals Cr, Cb are separately recorded. The color-difference signals are signals obtained by removing the luminance signal Y from three primary colors of R, G and B. As the color-difference signals, two signals of Cr=R-Y and Cb=B-Y are employed. For example, the sampling frequency of the luminance signal Y is 13.5 MHz and the sampling frequency of each of the color-difference signals Cb, Cr is 6.75 MHz. Therefore, the ratio of sampling frequency of these signals is 4:2:2.
On the other hand, with respect to picture data in conformity to the above-described digital VTR standard for commercial use, the sampling frequency ratio of the luminance signal Y and the color-difference signals Cb, Cr is expressed as 4:1:1 in the NTSC system (with 525 lines) or 4:2:0 in the PAL system (with 625 lines). That is, by reducing the sampling frequency of the two color-difference signals to half the sampling frequency of the luminance signal, the information amount of picture data to be recorded on a magnetic tape is reduced. Thus, in accordance with the digital VTR standard for commercial use, a signal is generated such that the band of a chroma signal of a base band signal is narrowed, and digital compression by intra-frame DCT is carried out to compress the picture data quantity to approximately 1/5 for recording.
As described above, the picture data compression systems (formats) which differ from one another are used for the digital VTR, and there is no digital VTR which can handle plural data compression formats. Therefore, for example, when the compressed picture data of 4:1:1 and the compressed picture data of 4:2:2 as described above are to be edited, editing operation must be carried out after temporarily expanding either one of the compressed picture data, and deterioration in picture quality generated in re-compressing the edited picture data cannot be avoided.
In addition, with respect to the digital VTR for broadcasting business in which high picture is required, higher picture quality exceeding the performance of the conventional digital VTR is required in some cases. For example, a base band of 4:2:2 is required, or a picture having less compression noise is required. Also, with respect to the digital VTR for commercial use, adoption of a standard for higher picture quality is expected.